For low-noise operation with high linearity, a digital-to-analog converter (DAC) requires a clean reference voltage. However, conventional DACs may suffer from a frequency-dependent distortion resulting from a disturbance or glitch of its reference voltage(s). In particular, as the DAC is driven to produce an output sinusoid, the frequency-dependent glitch on its reference voltage produces undesirable harmonic output frequencies in addition to the desired output sinusoid. For example, a conventional resistive DAC (RDAC) 100 including a plurality of RDAC stages 115 is shown in FIG. 1 (for illustration clarity, just one RDAC stage 115 is shown in FIG. 1) that uses both a positive reference voltage (Vrefp) and a negative reference voltage (Vrefn). A buffer 105 generates the positive and negative reference voltages. The positive reference voltage is carried on a positive voltage rail whereas the negative reference voltage is carried on a negative voltage rail.
Each RDAC stage 115 includes a switch matrix 130 formed by a switch S1, a switch S2, a switch S3, and a switch S4. Switch matrix 130 has two complementary states as controlled by a bit C and its complement Cb. Depending upon the state for switch matrix 130, the positive voltage rail couples through a corresponding resistor R1 to either a non-inverter input 135 or an inverting input 140 of an amplifier 125 in an amplifier stage 120 such as for a headphone (HPH). The negative rail couples through the remaining resistor R1 to whatever input of amplifier 125 that is not coupled to the positive voltage rail. The various bits input to RDAC stages 115 represent a digital word that is converted by RDAC 100 into a corresponding analog output signal from amplifier 125.
Suppose that a succession of the digital words codes for a sinusoidal output signal at some fundamental frequency. At each new digital word, some of the RDAC stages 115 will switch their switching state. For example, instead of charging non-inverting input 135 with the positive reference voltage, the corresponding switch matrix 130 is instead switched so that the positive reference voltage charges inverting input 140. Every time an RDAC stage 115 switches between its switching states, the parasitic capacitances for its switch matrix 130 causes a glitch or disturbance on the positive and negative reference voltages. This glitch is relatively small if only a few RDAC stages 115 switch their switching state but is relatively large if more and more of RDAC stages 115 switch their switching state. At the peaks and valleys of the sinusoidal output signal, relatively few RDAC stages 115 switch whereas significantly more switch during the rising and falling edges for the sinusoidal output signal. The glitches on the reference voltages will thus rise in magnitude at twice the fundamental frequency for the sinusoidal output signal. The resulting harmonic disturbance of the reference voltages causes a corresponding harmonic disturbance of the sinusoidal output signal.
Accordingly, there is a need in the art for resistive digital-to-analog converters with stable reference voltages that are not subject to harmonic glitches.